This invention relates to an engine control system and method and more particularly to an improved feedback control system and method for a marine propulsion engine.
The control of an engine is extremely important in ensuring good running. In addition to providing the appropriate and desired output for the engine under a wide variety of running conditions, engine control are important in ensuring good fuel economy and also effective exhaust emission control. Therefore, it has been proposed to provide engines with sensors which sense the actual air-fuel ratio of the engine and make adjustments through a feedback control system so as to ensure that the air-fuel ratio is obtained at the desired amount.
Although this theory may be relatively simple, the application of this principle is not quite as straightforward. The problem is particularly acute in conjunction with marine propulsion engines. Unlike land vehicles, the exhaust gases from a marine propulsion engine are frequently discharged, under at least some running conditions, below the level of water in which the associated watercraft is operating. This underwater exhaust gas discharge is employed so as to assist in exhaust silencing.
With land vehicles the condition at the exhaust discharge generally is maintained fairly constant under all running conditions. That is, there are not outside factors which will affect the ability of the exhaust system to discharge the exhaust gases under most normal conditions. However, when the exhaust gases are discharged beneath the water, as with a marine propulsion engine, then the condition at the exhaust outlet can vary significantly.
One way in which the condition can vary is that the depth of the underwater exhaust gas discharge is not constant during the running of the watercraft that is powered by the engine. In fact, the variations are not even linear in relation to speed. The reason for this is that many watercraft use so-called "planing-type" hulls that operate fairly deeply submerged under low-speed conditions. However, as the speed increases and the watercraft goes on plane, the water level can change suddenly relative to the location of the exhaust gas outlet. This can have a significant effect on the performance.
It is, therefore, a principal object of this invention to provide an improved feedback control for a marine propulsion engine that takes into account the pressure at the exhaust gas discharge.
It is a further object of this invention to provide an improved feedback control for a marine propulsion engine wherein the depth of submersion of the exhaust outlet is factored into the feedback control variables.
In addition to the depth of the exhaust outlet in marine applications, the exhaust gases are frequently discharged, particularly under high-speed operation, through a through- the-propeller or through-the-hub exhaust gas discharge. However, when the watercraft is propelled by a propeller, it is the normal practice to mount the propeller so that its trim position can be adjusted. That is, the angle of axis of rotation of the propeller relative to the transom is varied. This is done both in outboard motors and in the outboard drive portion of an inboard/outboard drive. These types of drives are referred to generally as marine outboard drives.
In addition to changing the depth of submersion, the trim adjustment changes the angle at which the exhaust gases are discharged relative to the water level. Hence, the back pressure on the exhaust gases can vary with the trim angle, even if the depth is maintained uniform.
It is, therefore, a still further object of this invention to provide an improved feedback control for a marine propulsion engine wherein the trim angle of the drive is considered in setting the feedback control.
The types of feedback control employed generally control the air-fuel mixture by controlling the amount of fuel in response to the output of the sensor. The sensor is frequently an exhaust sensor, such as an O.sub.2 sensor, that emits a signal which is indicative of the richness or leanness of the mixture. If the feedback control is done only as a function of the output of this sensor, the control may not be as effective because it disregards the other factors noted above.
It is, therefore, a still further object of this invention to provide an improved feedback control system and method for a marine propulsion engine that takes into account exhaust back pressure and other factors which may affect it.
It has also been discovered that merely changing the amount of fuel supplied to the engine does not necessarily achieve the complete results desired. For example, with spark-ignited engines it has been found that the spark advance should also be altered in response to the feedback signal. Like the fuel-ratio variation, spark advance control also depends upon factors of exhaust gas pressure and other things that may affect it.
It is, therefore, a still further object of this invention to provide an improved feedback control system for the spark timing which is dependent upon factors affecting exhaust back pressure.
In controlling the air-fuel ratio, it is necessary, of course, to provide a good indication of air flow to the engine so that the fuel flow that is varied can be varied in proportion to air flow. With two-cycle engines, a type of engine frequently employed in marine propulsion applications, it has been noted that the amount of air flow to the engine can be accurately determined by measuring the pressure in the crankcase chamber at particular crank angles. Therefore, many engine control systems employ crankcase pressure sensors so as to control the amount of fuel supplied.
It has been discovered, however, that the exhaust back pressure also can vary the accuracy of the crankcase pressure sensor in determining the accurate airflow to the engine.
It is, therefore, a still further object of this invention to provide an improved feedback control system where the airflow is measured by crankcase pressure and wherein adjustments are made in the air flow determination based upon factors which affect the back pressure in the exhaust system.
In engines that have plural cylinders, frequently the engine is supplied with an exhaust system that includes a manifold that collects exhaust gases from a number of exhaust ports and delivers it to the atmosphere through a common exhaust gas opening of the exhaust system. With these systems and with particular applications the distance between the exhaust gas opening and the exhaust ports of the individual cylinders may be different. This is a problem that is particularly acute in conjunction with marine propulsion applications due to the compact nature of the exhaust system that must be employed for these applications. This difference in length can be particularly significant in conjunction with two-cycle engines, wherein exhaust pulses can have a significant effect on the charging of the individual cylinders. This is caused in part by the substantial overlap between the opening of the scavenger port and the closing of the exhaust port. When utilizing a feedback control, the collected flow of the exhaust gases is normally measured, and this is used for determining the air-fuel ratio. As a result, although the average for the system may be acceptable, individual cylinders are not supplied with the appropriate air-fuel mixture.
It is, therefore, a still further object of this invention to provide an improved feedback control system for an engine having plural cylinders served by a common exhaust system and wherein the feedback control is varied on a cylinder-by-cylinder basis.
It is frequently the practice to use a common basic engine architecture for a family of engines. The engines within the family provide different power outputs or different performance, by varying the accessories and components associated with the engine. Although this has advantages, it also has disadvantages. For example, it is necessary for the manufacturer, distributors, and dealers to stock a wide variety of parts to serve different engines of a common family. In addition, it makes it difficult to judge and meet consumer preferences for varying desires. That is, more engines of one particular type may be desired in one year than in another year.
It is, therefore, a still further object of this invention to provide a control system for an engine wherein the basic structure of the engine is the same and its performance is varied only by the control strategy employed so as to provide a family of engines which is substantially identical in structure.